This invention relates to a process for treating the surface and near surface regions of a polymer with high intensity pulsed ion beams with sufficient beam fluence to achieve the various effects of cross-linking, pyrolizing, etching or ablating the polymer in the treated areas. More particularly, the ion beam pulses are characterized by pulse widths of less than 10 microseconds per spatially contiguous pulse, beam intensities of 0.01 to 10 J/cm.sup.2, and ion energies of typically greater than 25 keV.
The use of beams of high energy particles or photons for modifying polymers has been known and practiced for years using sources of particles such as the radiation decay products of radioactive elements (e.g. .sup.60 Co) and electron beams produced from continuous and pulsed beam sources. Typical ion beam polymer treatment uses sources of high energy ions from expensive, research-type accelerators such as linear accelerators (linacs) or Van de Graff accelerators that are expensive, produce very low dose rates and, although useful as diagnostic tools for research, are not suitable for commercial treatment. Electron beams (typically &gt;1 MeV electrons) are used to deliver dose rates to polymers up to several hundred Mrad/hr. Photons (10-30 eV) have also been used to treat polymer surfaces by inducing chemical reactions. Continuous, high energy photon (.gamma.) sources (e.g. .sup.60 Co with 1-3.5 MeV photons) are most commonly used for commercial irradiation of polymers. These techniques have demonstrated the ability of high energy particles at dose levels of order 10-100 Mrad to produce beneficial changes to polymers including improved toughness, resistance to solvents, and increased adhesion, as well as changes in optical density and electrical conductivity.
Although present techniques have been shown to be valuable in many polymer treatment applications, these treatment methods have several shortcomings.
The treatment typically extends deep into the material (e.g., the range of 1 MeV electrons is approximately 0.5 cm, the range of MeV photons is &gt;&gt;1 cm). This relatively deep treatment requires large total treatment doses to produce a significant effect. This occurs because of the difficulty in obtaining high fluences of low energy particles using existing treatment methods and the problem of surface heating that results from high continuous irradiation levels.
Any of the techniques described above for irradiation of polymers will produce reaction products, such as free radicals, ionized molecules, and broken bonds, along the regions caused by energy deposition from particles moving through the polymer. Interaction between such reaction products would both increase the rate of the expected chemical reactions within the polymer (such as cross-linking) and enable unusual reactions normally precluded by the relative stability of the carbon-carbon bond. However, the low dose rates available using present technology precludes such interactions, as the density of reaction products is far too low. Even if the total dose is large enough to produce closely spaced tracks (taking minutes to hours), the time between the creation of adjacent tracks is longer than the typical recombination lifetimes of many of the reaction products (&lt;1 microsecond for ions and excited states, &gt;1 .mu.s and longer for free radicals). This means that the advantages of increased interactions between reaction products resulting from high densities of reaction products cannot be realized using present techniques.
Present techniques also cannot produce dose rates sufficient to produce effective pyrolyzation (removal of hydrogen and oxygen) or etching (removal of material by rapidly heating the surface material beyond the point at which it begins to vaporize) of polymer surfaces without significantly affecting the underlying material. In addition, high pulsed doses of particles delivered to polymer surface regions can also modify the topology of the near surface region (e.g. by producing a rougher surface texture) in a way that present technology cannot support.